Optical communication systems employing optical fibers are an important technology for transmitting a large amount of data over a long distance. Optical communication systems that are currently in use are of the intensity modulation type. The intensity modulation process is a modulation process for assigning the “presence” and “absence” of optical pulses to “1” and “0” of digital signals. The intensity modulation process is widely used in a wide range of applications because it is easy to generate and detect modulated signals and it is possible to transmit modulated signals over a long distance.
As recent years have seen the transmission of a growing amount of information, the optical communication systems have been required to have a high-speed transmission capability. The optical communication systems that are currently in use have a data transmission rate of up to about 10 Gbps. For optical communication systems of the next generation, research efforts are being made to achieve a data transmission rate of about 40 Gbps. In addition, there is a strong demand for a reduction in the cost in view of increased transmission distances. Technologies for transmitting data over distances in excess of 1000 km are also being studied.
There are known two tasks to be accomplished in order to realize high-speed, long-distance optical communication systems.
The first task is to deal with increasing optical noise.
If an intensity-modulation optical communication system has a high data transmission rate, then it suffers from a large amount of noise because the transmission band which the system utilizes is wide. As a result, the signal to noise ratio at the signal reception end is lowered, resulting in increased code errors and lowered communication quality. A longer transmission distance requires the system to have a greater number of repeaters for amplifying optical signals to compensate for a loss of optical intensity. Optical noise generated by optical amplifiers incorporated in the repeaters accumulates to lower the signal to noise ratio at the signal reception end. For realizing high-speed, long-distance optical communication systems, therefore, it is necessary to develop a transmission system which is capable of reducing optical noise or resistant to optical noise.
In recent years, attention has been focused on the phase modulation principle, in particular the DPSK (Differential Phase Shift Keying) principle, applied to optical communication systems as a countermeasure against optical noise. The DPSK process is one of phase modulation processes for expressing information with a combination of waves that are kept out of phase with each other. According to the DPSK process, whether an optical signal is of “1” or “0” is expressed by its phase relationship to a preceding optical signal that has been sent (a signal in a preceding bit slot). Particularly, an optical communication system based on a combination of the DPSK process and a 1-bit delay detecting reception process is of high performance and can be of a simple construction.
In the system based on the combination of the DPSK process and the 1-bit delay detecting reception process, when transmission data is of “1”, the transmission side changes the phase of the bit slot by 180° and transmits the data, and when the transmission data is of “0”, the transmission does not change the phase of the bit slot and transmits the data.
The reception side divides the received signal, delays one of the divided signals with a 1-bit delay device, and causes the delayed signal to interfere with the other divided signal. At this time, if the signal in a preceding bit slot and the signal in a next bit slot are in phase with each other, then the interference signal has a maximum intensity level. If the signal in the preceding bit slot and the signal in the next bit slot are 180 degrees out of phase with each other, then the interference signal is extinguished. Based on this principle, the system based on the combination of the DPSK process and the 1-bit delay detecting reception process converts information expressed by a phase change into intensity information.
Using the DPSK process makes it possible to transmit data with less errors even in a reception state where the signal to noise ratio is low, than with the intensity modulation process. The reasons for this reduced-error data transmission will be described below.
FIGS. 1(a) and 1(b) are graphs showing the distances between codes “1” and “0” on a complex electric field plane. FIG. 1(a) shows the positional relationship between the codes “1” and “0” according to the intensity modulation process. FIG. 1(b) shows the positional relationship between the codes “1” and “0” according to the DPSK process.
As can be seen from FIGS. 1(a) and 1(b), the distance between codes “1” and “0” on the complex electric field plane according to the DPSK process is twice the distance between codes “1” and “0” on the complex electric field plane according to the intensity modulation process. According to the DPSK process, therefore, the same code error rate as according to the intensity modulation process is obtained even if the amount of noise is twice, i.e., even if the signal to noise ratio is 1/2. The DPSK process is thus resistant to noise and lends itself to making optical communication systems higher in transmission rate and longer in transmission distance.
The second task to be accomplished in order to realize high-speed, long-distance optical communication systems is concerned with a countermeasure against optical waveform distortions.
One major factor for causing optical waveform distortions in optical communication systems is a nonlinear optical effect of optical fibers. It is known that according to the intensity modulation process, waveform distortions caused by the nonlinear optical effect increase as the transmission rate becomes higher. It is also known that waveform distortions caused by the nonlinear optical effect pose a big problem on long-distance data transmission. In order to realize high-speed, long-distance optical communication systems, therefore, it is necessary to use optical fibers with a small nonlinear optical effect or to use a transmission process which is resistant to the nonlinear optical effect.
To accomplish the second task, Japanese Patent Laid-Open No. 2003-060580, for example, has proposed a process for using an RZ (Return to Zero) pulse for each bit of the DPSK signal. This process is called an RZ-DPSK process. According to the RZ-DPSK process, waveform distortions are suppressed by two advantages obtained by using an RZ pulse for each bit of the DPSK signal.
The first advantage is that since the optical intensity of peaks becomes greater than the average optical intensity by using RZ pulses, the signal to noise ratio is improved to make it possible to transmit data with lower optical intensity. The second advantage is that interbit pulse interference can be reduced by using RZ pulses. In view of these advantages, the RZ-DPSK process has quickly been recognized in recent years as a process for transmitting data at a data transmission rate of 40 Gbps over long distances.
According to the RZ-DPSK process, as described in a non-patent document (A. H. Gnauck, S. Chandrasekhar, J. Leuthold, L. Stulz, “Demonstration of 42.7-Gb/s DPSK Receiver With 45 Photons/Bit Sensitivity”, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 1, p. 99-101, January 2003), a receiver has a delay interferometer for converting a phase-modulated signal into a intensity-modulated signal. This system is referred to as a delay interference detecting system and is advantageous in that it can be reduced in size as no local oscillation light is required, compared with the known coherent reception system.
FIG. 2 is a block diagram showing a configurational example of a delay interferometer called a Mach-Zehnder interferometer.
As shown in FIG. 2, the delay interferometer comprises first directional coupler 301 for dividing an input light, delay element 304 for delaying one of the divided lights, and second directional coupler 305 for coupling output lights from first and second arms 302, 303 through which the lights divided by first directional coupler 301 are propagated.
If a phase-modulated signal is input to the delay interferometer, then delay element 304 is set to an amount of delay corresponding to a time slot commensurate with one bit of the bit rate of the phase-modulated signal.
The light (the phase-modulated signal) input to the delay interferometer shown in FIG. 2 is divided by first directional coupler 301 into two lights of equal light intensity, which are input respectively to first and second arms 302, 303. Only the light which is input to first arm 302 is shifted in phase by 90 degrees (delayed by 90 degrees) by delay element 304.
Each of the lights which have been propagated through first and second arms 302, 303 is divided into two lights by second directional coupler 305. Second directional coupler 305 has a first output port (upper port in FIG. 2) that outputs the light intensity of ½ of the light propagated through first arm 302 and the light intensity of ½ of the light propagated through second arm 303. At this time, only the light propagated through second arm 303 is shifted in phase by 90 degrees (delayed by 90 degrees from the input light) by second directional coupler 305.
Since the light propagated through first arm 302 and the light propagated through second arm 303 are shifted out of phase with each other by 90 degrees, the first output port combines and outputs the lights that are in phase with each other.
Second directional coupler 305 has a second output port (lower port in FIG. 2) that outputs the light intensity of ½ of the light propagated through first arm 302 and the light intensity of ½ of the light propagated through second arm 303. At this time, the light propagated through first arm 302, which has been shifted in phase by 990 degrees by delay element 304, is further shifted in phase by 90 degrees by second directional coupler 305. As the light propagated through first arm 302 is shifted in phase by 180 degrees and the light propagated through second arm 303 is not shifted in phase, the second output port combines the lights that are in opposite phase with each other and hence outputs no light.
When the phase of the light that is propagated through one of the arms is thus adjusted in phase such that the two lights output from the first output port of second directional coupler 305 are in phase with each other, the two lights output from the second port are in opposite phase with each other. If a CW light (continuous wave light) is input to the delay interferometer, then the first port outputs the lights in phase with each other which intensify each other, and the second port outputs no light as the lights in opposite phase with each other cancel each other.
Operation of the delay interferometer at the time a DPSK signal is input thereto will be described below.
It is assumed that the DPSK signal is expressed by a code “0” represented by a light shifted in phase by 0 and a code “1” represented by a light shifted in phase by π.
The first output port of the delay interferometer outputs lights in phase with each other which intensify each other if bits that are adjacent to each other on the temporal axis are in phase each other, and cancel and extinguish lights if the phase difference between bits that are adjacent to each other on the temporal axis is π.
The second output port of the delay interferometer outputs extinguish lights if bits that are adjacent to each other on the temporal axis are in phase each other, and outputs lights in phase with each other which intensify each other if the phase difference between bits that are adjacent to each other on the temporal axis is π because the phase difference between the lights propagated through the two arms is 0 or 2π.
Therefore, if bits that are adjacent to each other on the temporal axis are in phase each other, then the first output port outputs a light, and if the phase difference between those bits is π, then the second output port outputs a light. As a result, the phase information of the DPSK signal is converted into intensity information.
The delay difference between the lights which is caused by the propagation thereof through the two arms should preferably be equal to one time slot of the light signal such that only bits that are adjacent to each other on the temporal axis interfere with each other. If the delay difference deviates from one time slot, then an interferential component produced by interfering with another bit that is not to interfere with is introduced into the light signal output from the first output port or the second output port, generating a waveform distortion which tends to degrade the conversion from the phase information into the intensity information.
The RZ-DPSK process which uses the delay interferometer poses some problems.
The first problem is that if the delay interferometer is to receive a WDM (Wavelength Division Multiplex) light signal, for example, at a transmission rate that is currently employed as a standard rate, then the delay interferometer needs to be adjusted for each wavelength. The reasons will be described below.
The DPSK process has been developed for the purpose of being applied to optical communication systems having a transmission rate of 40 Gbps. According to ITU-T, therefore, two transmission rates of 39.81 Gb/s and 43.01 Gb/s have been determined as standard rates, and many systems are considered to employ the transmission rate of 43.01 Gb/s.
The ITU-T standards specifies that WDM optical communication systems shall multiplex information at a frequency interval of 100 GHz, and may systems employ this frequency interval.
For converting a DPSK signal having a transmission rate of 43.01 Gbps into an intensity signal using the delay interferometer, the delay difference between lights propagated through two arms may be set to one time slot, i.e., about 23.3 ps.
FIG. 3 shows the dependency on the frequency of light intensities that are output from the first output port and the second output port when a CW light is input to the delay interferometer thus adjusted. The vertical axis of the graph shown in FIG. 3 represents the light transmittance of interference lights propagated and output through the two arms and the output ports, and the horizontal axis the relative frequency at the time the WDM central frequency serving as a reference frequency is nil.
As shown in FIG. 3, the interval between the peaks of the light intensity of the interference lights that are output from the two output ports, i.e., the interval between interferential frequencies, is 43.01 GHz.
An example will be described below in which the delay difference between lights propagated through the two arms of the delay interferometer is adjusted to obtain peaks of the interference lights output from the output ports at frequency 401-c shown in FIG. 3.
At frequency 401-c, the output lights from the first and second arms of the delay interferometer and a differential circuit output representative of the difference between those output lights have good waveforms 503 shown in FIG. 4. At adjacent frequencies 401-a, b, d, e, the DPSK signal cannot properly be converted into an intensity signal as these frequencies deviate from the frequencies at which the interference lights have peaks. Specifically, as indicated by waveforms 501, 502, 504, 505 shown in FIG. 4, the amplitudes of the lights output from the arms are reduced in level and the waveforms thereof are distorted, resulting in a degraded reception capability. Particularly at frequencies 401-a, b, d, e, since frequency deviations from the frequencies at which the interference lights have peaks are different from each other, the reception capability differs from frequency to frequency.
Consequently, in order for WDM optical communication systems to convert a DPSK signal into an intensity signal using the delay interferometer, it is necessary to make fine adjustment of the delay difference between lights propagated through the first and second arms at each of frequencies (hereinafter also referred to as central frequencies) used by the WDM process, for thereby causing the interference lights to peak at the respective central frequencies. However, such adjustment is so complex as to increase the cost required to adjust the system.
The second problem is that the above scheme for adjusting the delay interferometer at each of the frequencies makes it difficult to keep stable the frequencies at which the interference lights are peaked.
As described above, in order for WDM optical communication systems to achieve a data transmission rate of 43.01 Gbps at a frequency interval of 100 GHz, it is necessary to adjust the delay difference between lights propagated through the two arms of the delay interferometer at each central frequency. Such fine adjustment can be performed by a method of slightly positionally moving mirrors disposed on the arms or the like with piezoelectric devices or the like, or a method of making the arms as quartz waveguides and adjusting the waveguide characteristics of the arms based on a thermooptical effect.
However, if the delay interferometer is equipped with an adjusting mechanism, i.e., a mechanism for varying the frequencies at which the interference lights are peaked, then the operating frequencies tend to become unstable after the adjustment. In particularly, the method of adjusting the delay difference based on the thermooptical effect is difficult to maintain stability because it is susceptible to changes in the ambient temperature. As a result, the reception capability is possibly degraded.